1. Field of the Invention
The present invention relates to a piezoelectric element drive method and also to a liquid ejecting device for ejecting liquid such as ink.
2. Description of the Related Art
Liquid ejecting devices for ejecting liquid such as ink to record an image on a recording medium generally have a liquid ejection head for ejecting liquid that is mounted thereon.
A mechanism that utilizes a pressure chamber whose capacity can be reduced when a piezoelectric element that operates as part or all of the walls thereof is deformed is known as mechanism for ejecting liquid from a liquid ejection head. With such a mechanism, the piezoelectric element is deformed by applying a voltage to the element and making the pressure chamber contract as a result of the deformation of the piezoelectric element to consequently eject liquid from the pressure chamber by way of the ejection port formed at an end of the pressure chamber.
So-called shear mode type liquid ejection heads having pressure chambers, each of which has one or two of the inner walls that are formed by a piezoelectric element, is known as a type of liquid ejection head having a mechanism for ejecting liquid by means of piezoelectric elements.
A liquid ejection head of the shear mode type contracts the pressure chambers hereof by applying pressure to the piezoelectric elements thereof and hence not by elongation deformation and contraction deformation but by shear deformation.
Liquid ejecting devices for industrial applications are required to use highly viscous liquid. Then, such devices are required to produce large liquid ejecting power in order to eject highly viscous liquid. To meet the requirement, so-called Gould type liquid ejection heads having pressure chambers formed by using tubular piezoelectric members representing a circular or rectangular cross section have been proposed.
Each of the pressure chambers of a Gould type liquid ejection head is expanded or contracted by subjecting the piezoelectric member or members thereof to elongation deformation and contraction deformation in outward and inward directions (radial directions) relative to the center of the pressure chamber. Thus, in a Gould type liquid ejection head, all the wall surfaces of the pressure chambers thereof are deformed and the deformation of the wall surfaces serves to generate ejection force. Thus, a Gould type liquid ejection head can provide large ejection force if compared with a share mode type liquid ejection head.
The displacement amount of a piezoelectric element varies as a function of the bias voltage applied to it if the drive voltage waveform applied thereto remains same. For example, when a 10 Vp-p rectangular waveform is applied, the displacement amount varies between an instance where a rectangular wave from 0 V to 10 V (bias voltage=0V) is applied and an instance where a rectangular wave from 10 V to 20 V (bias voltage=10 V) is applied. Besides, liquid ejection heads manufactured with exactly same specifications can represent different displacement characteristics of piezoelectric elements thereof relative to the bias voltage applied to them. In other words, the liquid ejection characteristics can vary from a liquid ejection head to another if a same bias voltage is used.
In view of this problem, Japanese Patent Application Laid-Open No. 2006-321200 discloses a technique of selecting a bias voltage that maximizes the displacement amount of a piezoelectric element by changing the bias voltage being applied to the piezoelectric element and thereby detecting the displacement characteristics of the piezoelectric element relative to the bias voltage.
The above-cited technique can obtain large ejection force because a bias voltage that maximizes the displacement amount of a piezoelectric element can be selected and applied to the piezoelectric element.
The use of a piezoelectric element that is made of a material referred to as a soft material and can produce a large displacement amount has been and is being studied in order to obtain large ejection force. In comparison with hard materials, however, soft materials generally represent a record of large displacement amounts (a large hysteresis curve) when the voltage being applied to the material is changed. Additionally, soft materials represents a low coercive electric field if compared with hard materials. Thus, when a piezoelectric element referred to as soft material is employed, degradation of polarization characteristics can arise to change the displacement characteristics of the piezoelectric element when a negative voltage is applied thereto.
With the technique disclosed in Japanese Patent Application Laid-Open Publication No. 2006-321200, if a bias voltage is selected so as to maximize the displacement amount of a piezoelectric element, the displacement amount of the piezoelectric element can be changed to consequently give rise to unstable ejection force because of degradation of the hysteresis curve and that of polarization characteristics. This problem becomes remarkable when a soft material having the above-described characteristics is employed.